This invention is directed to a system and method for automatically crowbaring a circuit for the purpose of quenching an arc in a load device or limiting the arc energy, by applying a crossed-field switch device in parallel to the load device.
Crossed-field electrical discharge devices are devices with two facing electrodes having magnetic and electric fields at an angle to each other. Gas in the interelectrode space is subject to cascading ionization with the magnetic field is on, but the path length of the particles in the interelectrode space is too short for cascading ionization when the magnetic field is below a critical value, so the device becomes nonconducting. Thus, it can be arranged to function as an off-switch by reducing the magnetic field.
Furthermore, with the normal, above critical magnetic field and a high voltage applied thereto, the crossed-field switch device is non-conductive because the high interelectrode potential maintains a short charged particle path even in the presence of the magnetic field. However, when the applied potential is reduced, at a particular point where cascading ionization can commence, the device automatically becomes conductive. With this arrangement, the crossed-field switch device automatically acts as a crowbar, to short-circuit the load when the potential across the load fails.
Crossed-field electrical discharge devices were primarily laboratory curiosities, until recent developments have shown that they are capable of carrying fairly high direct currents and offswitching against fairly high voltages. This capability has resulted in their design into a number of circuit breakers. In such circuit breakers, the crossed-field devices become crossed-field offswitching devices which perform the function of offswitching current to result in increasing circuit breaker impedance. Prior patents which can use suitable cross-field switch devices as their switching elements in circuit breaker environments include K. T. Lian Pat. No. RE 27,557; K. T. Lian and W. F. Long Pat. No. 3,641,358; M. A. Lutz and W. F. Long Pat. No. 3,660,723. These illustrate the manner in which a crossed-field switch device can be used.
Two patents which illustrate particular structure of a crossed-field switch device are G. A. G. Hofmann and R. C. Knechtli Pat. No. 3,558,960 and M. A. Lutz and R. C. Knechtli Pat. No. 3,638,061. These patents discuss the maintenance of pressure in the interelectrode gap and during conduction. Gunter A. G. Hofmann Pat. No. 3,769,537 teaches the use of a perforated inner electrode to supply gas to the gap. Furthermore, G. A. G. Hofmann Pat. No. 3,604,977 and M. A. Lutz and G. A. G. Hofmann Pat. No. 3,678,289 discuss the management and control of the offswitching of crossed-field switch devices by control of the magnetic field.
M. A. Lutz and R. Holly Pat. No. 3,890,520 teaches a field emission device for supplying initial electrons to the interelectrode space. J. R. Bayless and R. J. Harvey Pat. No. 3,949,260 uses a thin wire discharge for that purpose. Michael A. Lutz and Gunter A. G. Hofmann Pat. No. 3,876,905 teaches a particular electrode structure for crossed-field devices. Gunter A. G. Hofmann Pat. No. 3,873,871 relates to the physical structure of the crossed-field switch device and particularly the arrangements of the magnetic field windings. Continuing improvements are being made to enhance the voltage and current capabilities, as well as life and reliability of the crossed-field switch devices.
The method of on-switching the crossed-field switch device of G. A. G. Hofmann Pat. No. 3,714,510 comprises the initiation of an interelectrode arc discharge which reduces the interelectrode potential, and after extinguishment of the interelectrode arc, the interelectrode potential is sufficiently low to initiate and permit conduction in the glow mode.
The patent application of R. J. Harvey, Ser. No. 797,720 filed May 17, 1977 is directed to the concept of on-switching by creating a high magnetic field in a localized area to permit a glow mode plasma to initiate in the localized area, with consequent reduction of the interelectrode voltage and spreading of the plasma so that glow mode plasma discharge takes place in the entire active area of the interelectrode space.